1. Field of the Invention
The present invention relates to crystal growth from solution.
As a bulk crystal growth technique for a compound semiconductor having a high vapor pressure, particularly group II-VI compound semiconductors, solution growth of crystals has been expected to become prominent because it can use a low growth temperature.
2. Description of the Related Art
Group II-VI compound semiconductors have a high melting point and a vapor pressure of its constituent elements is high. Therefore, a melt crystal growth chamber is required to be resistive against a high pressure, and in addition a crystal grown at a high growth temperature is likely to have a high density of crystal defects.
If solution growth is used, it is possible to lower a crystal growth temperature of II-VI group compound semiconductor, and a crystal of good quality can be expected. It has been proposed to use Group II or VI elements of a group II-VI compound semiconductor as a solvent.
FIG. 8 shows an example of the structure of a conventional solution growth apparatus for a group II-VI compound semiconductor. The left side of FIG. 8 shows a crystal growth apparatus in cross section, and the right side is a graph showing a temperature distribution in the crystal growth apparatus.
A crystal growth tube 1 is made of two quartz tubes laving different diameters. At the initial stage, the upper end of the tube 1 is open. A heat sink 6 made of a material having a high thermal conductivity is entered to the lower portion of the crystal growth tube 1. The material of the heat sink 6 has preferably a thermal conductivity about 100 times as high as quartz.
The heat sink 6 is fixed to the crystal growth tube 1. A seed crystal wafer 5 having generally the same diameter as the heat sink 6 is placed on the top surface of the heat sink 6. A tubular seed stopper 4 having a predetermined length and the same outer diameter as the heat sink 6 is inserted and fixed to the crystal growth tube 1. Before source material is filled in, the crystal growth tube 1 is open, for example, at its upper portion.
Se-Te (Se and Te of a predetermined mixing ratio, solid in a room temperature) as a solvent 3 and ZnSe polycrystal as a source crystal 2 are loaded in the crystal growth tube 1. IF Se only is used as the solvent for ZnSe crystal growth, solubility of ZnSe into Se is low. Therefore, Te is added to Se to increase the ZnSe solubility. After the source crystal 2 and solvent 3 are loaded, the crystal growth tube 1 is evacuated and the opening is hermetically sealed.
The crystal growth ampoule tube 1 prepared as above is placed in an electric furnace of an externally heating type set with a temperature gradient shown at the right side of FIG. 8. The externally heating type electric furnace has a furnace tube 7 and a heater wire 8 wound around the tube 7. The crystal growth tube 1 is placed in a vertical space off the furnace tube
The inside of the furnace tube 7 has a vertical temperature distribution high at the upper area and low at the lower area. The temperature at the position where the source crystal 2 is placed, is represented by Ts, and the temperature at the surface of the seed crystal 5 on which crystal grows, is represented by Tg, where Ts &gt;Tg.
With the crystal growth tube 1 provided with such a temperature distribution, the source crystal 2 at the high temperature area dissolves in the solvent 3 to the saturated solubility at the high temperature area. The saturated solubility at a high temperature is higher than at a low temperature. Source crystal compositions dissolved in the solvent 3 move to the lower temperature area by diffusion so that the solution at the lower temperature area becomes oversaturated.
As the seed crystal 5 at time lower temperature area contacts the oversaturated solution, a crystal grows on the surface of the seed crystal 5. In this manner, a bulk crystal grows on the surface of the seed crystal 5.
Another example of the structure of a crystal growth tube is shown in FIG. 9. In this example, an ingot seed crystal 5 is placed on a heat sink 6 having the same diameter as the seed crystal 5. The seed crystal 5 is fixed to the crystal growth tube 1 by denting the side wail of the tube 1.
A conventional growth method explained with reference to FIG. 8 is, however, not satisfactory in practical use in that the outer diameter of a grown crystal is smaller than that of the seed crystal, and that if a grown crystal is used later as a seed crystal, the outer diameter of a crystal to be next grown becomes further smaller.
If the ingot seed crystal shown in FIG. 9 is used and a growth speed of crystal is low, the thickness of a crystal grown once is approximately in the same order of that of the ingot seed crystal. It is therefore difficult to increase the number of crystals from one mother crystal and to improve the manufacture efficiency.
With the conventional crystal growth method explained with FIG. 8, the solvent moves lower to the side wall and bottom of the seed crystal if there is a small gap between the seed crystal and heat sink. The solvent moving to the bottom of the seed crystal may diffuse in the crystal, or dissolve the seed crystal at the high temperature area and precipate the dissolved crystal on the heat sink at the low temperature area. The solvent diffused in the grown crystal forms inclusions.
In the case of the conventional crystal growth apparatus shown in FIG. 8, the seed crystal on the heat sink is fixed by the quartz seed stopper 4. The thermal expansion coefficient of quartz is 0.5.times.10.sup.-6 /K which is smaller than 7.55.times.10.sup.-6 /K of ZnSe.
As a result, the expansion of the seed crystal and heat sink of the crystal growth tube in a high temperature atmosphere is greater than the quartz crystal growth tube. This expansion difference exerts a compression stress to the seed crystal at the position in contact with the seed crystal stopper 4. The crystal structure of the seed crystal is therefore degraded. The crystal grown on the degraded seed crystal has also a degraded crystal structure.